Microtubules are systems of very long tubular structures built from a helical wrapping of tubulin (Figure 8.19). Tubulin is involved in motility within cells, such as in the mitotic spindle, flagella, and nerve axons. Tubulin is composed of two subunits, and , each of molecular weight 55,000. They are present in equimolar amounts in tubulin as dimers.

Assembly of microtubules requires energy from the hydrolysis of GTP. One end of the microtubule grows more rapidly, and is called the plus end. The other end is called the minus end. Assembly of microtubules is depicted in Figure 8.20. The final assembly of a functional microtubule usually involves the binding of other proteins, called microtubule-associated proteins (MAPs) to its surface. MAPs may stabilize microtubule strucure and/or promote the association of microtubules into bundles.

The MAP called (tau) is found in neuronal tissue. Phosphorylation of dissociates from microtubules, destabilizing the microtubules. Hyperphosphorylation of has a much more dramatic effect, resulting in the formation of tangles of -filaments in neural axons, one of the major cellular symptoms of Alzheimer's disease.

The mitotic spindle is composed mainly of microtubules. The mitotic spindle contains microtubules that have a variety of functions (Figure 8.28). Some, called polar microtubules, extend between the centrioles, apparently pushing them apart. Others, the kinetochore microtubules, are attached to the kinetochores of the individual chromosomes and seem to pull the chromatids to the poles in telophase.